In an electroluminescence (EL) device, a light emitting layer is formed on a transparent substrate so as to be interposed between an anode and a cathode. When a voltage is applied between the electrodes, light is emitted by exciters generated by recombination of holes and electrons injected as carriers to the light emitting layer. EL devices are generally classified into organic EL devices in which an organic substance is used as an emitting material of a light emitting layer, and inorganic EL devices in which an inorganic substance is used as an emitting material of a light emitting layer. In particular, organic EL devices are capable of emitting light of high luminance with a low voltage, and various colors of emitted light are obtained therefrom depending on the types of emitting materials. In addition, it is easy to manufacture organic EL devices as planar light emitting panels, and thus organic EL devices are used as various display devices and backlights. Furthermore, in recent years, organic EL devices designed for high luminance have been realized, and attention has been paid to use of these organic EL devices for lighting apparatuses.
FIG. 4 (sic. correctly FIG. 5) shows a cross-sectional configuration of a common organic EL device. In an organic EL element 101, a translucent anode layer 103 is located on a translucent substrate 102, and an organic layer 104 which is made up of a hole injection layer 141, a hole transport layer 142, and a light emitting layer 143 is located on the anode layer 103. A light reflective cathode layer 105 is located on the organic layer 104. When a voltage is applied between the anode layer 103 and the cathode layer 105, light, which is emitted by the light emitting layer 143 of the organic layer 104, passes through the anode layer 103 and the substrate 102 and then is taken out to the outside of the element.
In such an organic EL element 101, when only one type of the emitting material is used for the light emitting layer 143, an emission spectrum of output light has only one maximum value. However, it is necessary to obtain an emission spectrum having plural maximum values to output white light used for the lighting apparatuses or the like. Thus, there is a known organic EL element in which one light emitting layer is doped with plural emitting materials having different emission spectra, respectively (for example, refer to the forefront of organic EL technology development, pp. 24 to 25, Technical Information Institute Co., Ltd, abbreviated as non-patent document hereinafter). Moreover, there is also a known organic EL element which is provided with plural light emitting layers having different emission spectra, respectively.
However, in the organic EL element described in the above non-patent document, the respective emitting material have different life durations depending on their types, use conditions, or the like, so that once an emission of the emitting material which emits light of a predetermined wavelength reduces due to the life-span, the emission of the other emitting materials relatively increases. The white light is generated by mixing the light outputted from the respective emitting materials in a predetermined balance, so that when the emission of any emitting material reduces or increases, the balance of the emission is lost and thus the desired white light cannot be obtained. That is to say, in the organic EL element described in the above non-patent document, the period during which the white light can be obtained depends on the emitting material which has the short life-span. Also when the plural light emitting layers is formed, in the same manner as the above, the period during which the white light can be obtained depends on the emitting material which has the short life-span. Moreover, it is not easy to closely control film thicknesses of the light emitting layers, respectively, and furthermore, plural film forming processes are necessary, so that there is a problem that manufacturing efficiency is reduced.